In two separate studies this week, researchers show that exposure to air pollution can boost the odds of heart attack or stroke — by as much as 5% and 34%, respectively.

Previous studies on air pollution and heart attack risk haven’t been very conclusive; some showed a positive link while others found no association and still others suggested that only select pollutants appeared to increase the risk of heart problems. To get a clearer sense of the data, Dr. Hazrije Mustafic, a cardiologist at University Paris Descartes, and her colleagues searched for all previous studies investigating air pollution and heart attacks, and winnowed the list down to the 34 that reliably measured exposure to pollutants and heart events occurring within a day later.

Overall, the researchers found that people exposed to higher levels of particulates and polluting gases, which include carbon monoxide, nitrogen dioxide and sulfur dioxide, were anywhere from 0.5% to 5% more likely to experience a heart attack on the day of that exposure or on the following day.

The risk for healthy people may be relatively low, but consider that unlike other factors that contribute to heart disease — such as smoking, high blood pressure and high cholesterol — almost everyone breathes polluted air.

“On an individual level, the relative risks are rather low,” says Mustafic, lead author of the paper published in the Journal of American Medical Association. “But if you consider the population view, the entire population in industrialized countries is exposed to air pollution, so the impact on public health is not negligible.”

Exposure to polluted air can damage the heart system in a variety of ways, starting by increasing inflammation, which can destabilize plaques in the heart arteries and trigger heart attacks. Toxic compounds in the air can also increase heart rate and make the heart less responsive when it needs to pump more or less blood, and slow down the circulatory system as a whole by making the blood thicker and more viscous.

In a separate study led by Gregory Wellenius at Brown University, scientists found that exposure to elevated levels of pollutants typically emitted by cars and trucks could trigger a stroke within hours. Even more disturbing was the fact that the risky levels were lower than the U.S. Environmental Protection Agency’s danger zone for poor air quality; on all the days included in the study, the air quality, measured by the level of fine particulates from car exhaust, wood burning and power plant or manufacturing emissions, was considered satisfactory or moderate, meaning it posed potential health problems only to a small group of people.

To measure the link between air quality and stroke risk, researchers looked at patients who were admitted with stroke at Beth Israel Deaconess Medical Center between 1999 and 2008 and then correlated their hospital admissions with fine particulate readings from the Harvard School of Public Health’s ambient air monitoring station, located near the hospital campus and within 13 miles of 90% of the stroke patients’ homes.

According to the data, the risk of stroke was 34% higher after a 24-hour period of moderate air quality, in which concentrations of fine particulates measured 15-40 micrograms per cubic meter of air, compared with periods following a day of good air quality, in which the particulate concentration was less than 15 micrograms per cubic meter of air. Peak stroke risk occurred at about 12 hours after exposure to moderate quality air.

Wellenius and his colleagues also found that the danger was greatest for those exposed to black carbon and nitrogen dioxide (NO2), both emitted from fuel-burning car engines. Previous studies have found that short-term exposure to NO2 can trigger respiratory problems, including inflammation in the airways, in healthy people, and that concentrations of NO2 are anywhere from 30% to 100% greater near roads than away from roadways.

The fact that the stroke risk was up to 34% greater even on days that the EPA considers relatively safe for most people raises concerns that the agency’s standards may not be rigorous enough to protect public health, says Dr. Murray Mittleman, an associate professor of medicine at Harvard Medical School and senior author of the paper, published in the Archives of Internal Medicine. “Any proposed changes in regulated pollution levels must consider the impact of lower levels on public health,” he wrote in a release describing the study.

Mustafic notes also that her group’s findings should influence environmental policy on acceptable levels of air pollution. “The relative risk is rather low but the prevalence of exposure — everyone is exposed — is very, very important,” she says. “The exposure does not raise risk as much as tobacco, but 20% of people in France are exposed to tobacco but 100% are exposed to air pollution.” And even small risks from exposure, she says, can add up.